Peptides are compounds formed by the condensation of amino acids. The peptides exhibit various functions in vivo and are therefore expected to be applied as drugs. The peptide drugs are expected to exert various novel excellent functions by the examination of a novel amino acid sequence or the introduction of a non-natural amino acid, whereas the peptide drugs present problems such as rapid degradation by various enzymes in the body. In contrast to this, the introduction of disubstituted amino acids with their α-carbon atoms each substituted by two substituents to peptides has been found to be able to enhance the stability of the peptides against peptidases in vivo.